1. Field of the Invention
The present application is directed to electromagnetic rail-guns and, more particularly, to projectiles launched from electromagnetic rail-guns and the conducting rails used in the rail-gun system.
2. Description of Related Art
Electromagnetic rail-guns utilize an electromagnetic force called the Lorentz force to propel an electrically conductive integrated launch package (ILP). In a typical electromagnetic rail-gun, the ILP slides between two parallel rails and acts as a sliding switch or electrical short between the rails. By passing a large electrical current down one rail, through the ILP, and back along the other rail, a large magnetic field is built up behind the ILP, accelerating it to a high velocity by the force of the current times the magnetic field. An electromagnetic rail-gun is capable of launching an ILP to velocities greater than fielded powder guns, thereby achieving greater ranges and shorter flight times to engagement.
A conventional prior art electromagnetic rail-gun utilizes two long parallel rails capable of carrying a large current. A sliding, conducting armature is positioned between the two rails. The armature is adapted to slide between the two rails along their entire length. Application of a voltage across two ends of the two rails causes a large current pulse to flow through one rail, through the armature, and into the other rail. The current generates a magnetic field. The Lorentz force created by the interaction of the magnetic field with the current in the armature causes the armature to be rapidly propelled between the two rails in a direction away from the points of application of the voltage. The armature itself may be projected like a bullet at a target, or the armature may be used to push a bullet-type projectile (ILP) at high velocity towards a chosen target.
A disadvantage of the conventional rail-gun is that arcing and heating may occur between the armature and rails. The heating is due to I2R losses and the arcing is due to poor contact between the armature and rails.
Maintaining good electrical contact between the armature and the rails over the entire length of the rails without causing too much friction is a serious problem that has impeded rail gun development to date. If the contact between the armature and rails is too tight, friction slows the armature, metal fusion occurs, and degrades projectile velocity. If the contact between the armature and rails is too loose, arcing occurs. Significant damage to the rails can occur due to the friction of the armature, metal fusion, and arcing. Damage to the rails results in limited life for the EMRG rails necessitating replacement of the rails.
The early electromagnetic rail guns incorporated a solid armature that was propelled between the rails by the electromagnetic force generated by the current flow through the armature and the rails. However, it was soon found that at high speeds around one kilometer per second, the rails and armature were substantially damaged, possibly as a result of ohmic heating and/or internal forces. Further, increases in current flow tended to increase rail and armature gouging.
Current rail gun armature materials are less than optimal because the conditions encountered during launch deteriorate the armatures, which leads to degradation of the rails. Aluminum alloys in the 6000 and 7000 series are the most commonly used materials for armatures. Thermal stresses, caused by loss of armature/rail contact, typically dominate armature deterioration. One theory of the physics of rail gun launch is that when the current pulse decreases from peak level, there is a local magnetic force field reversal in the armature leg, leading to the loss of contact pressure. Because of this local reversal, the magnetic force field is no longer able to counteract the blow-off force generated by the current through the armature-rail contact. At this point in time, the aluminum armature material has been heated to very high temperature, exceeding the melting point of the aluminum at the armature/rail contact. With the elevated temperature, the aluminum's mechanical properties are no longer sufficient to prevent the legs of the armature from moving away from the rail. This loss of contact resulting in increased arcing and further melting of the armature material. Evidence of the heat experienced by the armature can be seen in recovered armatures. Aluminum armatures frequently contain intragranular cracks after firing, suggesting high tensile stresses along the z-axis. Rapid, intense heating followed by surface cooling causes these discontinuities (1).
Rail damage comes in many forms. Some damage is caused at start up, such as rail damage in the form of axial grooves. The grooves are not associated with hypervelocity gouging or transition, but occur below 1 MA of current. At 1.4-1.7 MA, a single shot evidences this damage, but after 20 shots at 1.5 MA, the life of copper rails becomes limited. Localized electrical heating of the rail melts the armature aluminum, which erodes the rails at startup (2). The most troublesome damage is incurred during the transition from metal to metal contact to plasma contact during firing, which causes significant melting of the aluminum. The melted aluminum adversely reacts with the copper rails, forming aluminide and creating pitting and gouging in the rails.
Thus a new approach to armatures and rails is needed, one that enables repeated use of very high velocity projectile firings with minimal damage to the gun rails.